Field of the Invention
The invention concerns a method for creating at least two essentially parallel two-dimensional image data sets of a region of interest by operation of a magnetic resonance system having a coil array.
Description of the Prior Art
A basic problem for acquiring MR images is the scan time. This was initially reduced by software methods in the form of optimized pulse sequences, wherein the flip angle of the pulses, the number thereof, the setting of the gradients or the waiting times between individual sequence sections, was modified. It was thus possible to reduce the acquisition of a gradient echo image using the FLASH method from several minutes to a few seconds. Although this changes the contrast behavior, it remains T2*-dependent. The RARE method is well-known as a fast, spin echo based imaging method. Other methods such as GRASE or TrueFISP exist, which constitute a form of mixture of the basic methods.
In order to achieve a further reduction in the acquisition time, it has been proposed to use multiple coils for reading out the scan signal. Not all the k-space lines are acquired but only selected ones, but using the multiple coils. This is also known as undersampling. In order to prevent an aliasing artefact, i.e. foldover effects, that are caused by this procedure in the reconstructed image, different reconstruction algorithms are used that manage with fewer k-space lines and therefore the more time-consuming scanning (filling) of the k-space lines is unnecessary.
Such reconstruction methods are commonly referred to under the acronyms GRAPPA (GeneRalized Autocalibrating Partially Parallel Acquisition), SENSE (SENSitivity Encoding for fast MRI) and SMASH (SiMultaneous Acquisition of Spatial Harmonics).
In the case of SENSE, first presented in SENSE: sensitivity encoding for fast MRI. Pruessmann K P, Weiger M, Scheidegger M B, Boesiger P, Magn Reson Med., 42(5), 952-62, 1999, the coil sensitivities are measured and a pseudoinverse matrix is determined therefrom. Using this matrix, the acquired image data of all the coils is combined into a full image. In other words, the coil images are unfolded to produce a total view.
In the case of the GRAPPA reconstruction method, the missing k-space lines are reconstructed by determining a k-space line to be added from a number of scanned k-space, lines by mathematically shifting the measured signal in k-space.
These reconstruction methods are based on the fact that the coil sensitivities differ in the region of interest. In order to amplify or rather make optimum use of these sensitivity variations, several methods collectively referred to under the acronym CAIPIRINHA (Controlled Aliasing In Parallel Imaging Results IN Higher Acceleration) are known. These are based on the fact that the aliasings can be selectively varied during data acquisition.
In the case of MS-CAIPIRINHA (Felix A. Breuer, Martin Blaimer, Robin M. Heidemann, Matthias F. Mueller, Mark A. Griswold, and Peter M. Jakob: Controlled Aliasing in Parallel Imaging Results in Higher Acceleration for Multi-Slice Imaging, Magn. Res. Med. 53:684-691, 2005), two slices are excited by alternating dual band pulses.
2D-CAIPIRINHA (Felix A. Breuer, Martin Blaimer, Matthias F. Mueller, Nicole Seiberlich, Robin M. Heidemann, Mark A. Griswold, and Peter M. Jakob: Controlled Aliasing in Volumetric Parallel Imaging, Magn. Res. Med., 55:549-556, 2006) is based on improving sensitivity variations for three-dimensional imaging in the phase encoding directions then present in two spatial directions.
In blipped CAIPIRINHA (Blipped-Controlled Aliasing in Parallel Imaging for Simultaneous Multislice Echo Planar Imaging With Reduced g-Factor Penalty. Kawin Setsompop, Borjan A. Gagoski, Jonathan R. Polimeni, Thomas Witzel, Van J. Wedeen, and Lawrence L. Wald), the slice gradient is additionally switched in the form of blips, i.e. in an oscillating manner, during readout.
Although the sensitivity changes produced in CAIPIRINHA must be taken into account for the reconstruction methods, the usual and above mentioned methods, such as GRAPPA and SENSE, can be used. CAIPIRINHA changes the evaluation by the reconstruction taking place as if coils having other sensitivities were present.
For data acquisition, well-known methods such as TrueFISP can also be used for CAIPIRINHA. The difference lies in the number of acquired k-space lines and the sensitivity variation.
For acquisition of a three-dimensional volume, either three-dimensional data sets can be acquired. These have two phase encoding directions and their acquisition is time-consuming even when using parallel imaging. Or, for time-critical examinations, it is therefore preferable to acquire two-dimensional images in a plurality of slices. This is also known as multislice imaging. This type of data acquisition can also be speeded up using parallel imaging, cf. the remarks concerning MS-CAIPIRINHA.
In the case of spin echo methods, the slices can be acquired intermittently, i.e. one or more k-space lines of successive slices. This is repeated until a sufficient set of k-space lines is present in each slice.
Alternatively, each slice can also be acquired completely before the next follows. This is particularly the case for acquisition methods such as FLASH or TrueFISP or bSSFP (balanced steady state free precession).
The slices acquired in this way have therefore been acquired at different points in time. In the case of moving examination objects such as the heart or lung, the image data of the different slices are shifted relative to one another and must therefore be registered to one another in order to be able to create a 3D-image therefrom. For registration of the images, a high SNR is advantageous, for which reason bSSFP is preferred. However, with this type of data acquisition the achievable contrast is limited to the contrast obtained by means of bSSFP. This is a mixed contrast which depends on T1 and T2.
If the image data sets are to have other contrasts, it is possible, particularly in the case of moving examination objects, to register the scan signals using navigator echoes. However, the disadvantage of this is that the navigator echoes interrupt the sequence progression and the motion is characterized on the basis of few values, i.e. the characterization is prone to errors.